Name: Date: Core: About 25 percent of incoming sunlight is reflected by clouds, dust, and gases in the air. About 50 percent is absorbed by Earth s surface. This energy heats the land and water. 20 percent is absorbed by gases and particles in the atmosphere. 5 percent is reflected by the surface back into the atmosphere. Some absorbed energy is radiated back into the atmosphere. Energy in the Atmosphere: The sun s energy interacts with Earth s atmosphere and surface in several ways. About half is either reflected back into space or absorbed by the atmosphere. The rest reaches Earth s surface. Energy in the Atmosphere Before reaching Earth s surface, sunlight must pass through the atmosphere. The path of the sun s rays is shown in the picture above. Some sunlight is absorbed or reflected by the atmosphere before it can reach the surface. The rest passes through the atmosphere to the surface. Part of the sun s energy is absorbed by the atmosphere. The ozone layer in the stratosphere absorbs most of the ultraviolet radiation. Water vapor and carbon dioxide absorb some infrared radiation. Clouds, dust, and other gases also absorb energy. Some sunlight is reflected. Clouds act like mirrors, reflecting sunlight back into space. Dust particles and gases in the atmosphere reflect light in all directions, a process called scattering. When you look at the sky, the light you see has been scattered by gas molecules in the atmosphere. Gas molecules scatter short wavelengths of visible light (blue and violet) more than long wavelengths (red and orange). Scattered light therefore looks bluer than ordinary sunlight. This is why the daytime sky looks blue. When the sun is rising or setting, its light passes through a greater thickness of the atmosphere than when the sun is higher in the sky. More light from the blue end of the spectrum is removed by scattering before it reaches your eyes. The remaining light contains mostly red and orange light. The sun looks red, and clouds around it become very colorful.
Energy at Earth s Surface Some of the sun s energy reaches Earth s surface and is reflected back into the atmosphere. About half of the sun s energy, however, is absorbed by the land and water and changed into heat. When Earth s surface is heated, it radiates most of the energy back into the atmosphere as infrared radiation. As shown in the picture to the right, much of this infrared radiation cannot travel all the way through the atmosphere back into space. Instead, it is absorbed by water vapor, carbon dioxide, methane, and other gases in the air. The energy from the absorbed radiation heats the gases in the air. These gases form a blanket around Earth that holds heat in the atmosphere. The process by which gases hold heat in the air is called the greenhouse effect. Greenhouse Effect Sunlight travels through the Atmosphere to Earth s surface. Earth s surface then gives off infrared radiation. Much of this energy is held by the atmosphere, warming it. The greenhouse effect is a natural process that keeps Earth s atmosphere at a temperature that is comfortable for most living things. Over time, the amount of energy absorbed by the atmosphere and Earth s surface is in balance with the amount of energy radiated into space. In this way, Earth s average temperatures remain fairly constant. However, as you will learn later, emissions from human activities may be altering this process. Checkpoint Questions: 1) What happens to most of the sunlight that reaches Earth? 2) Why does the sky look blue? 3) What happens to the energy from the sun that is absorbed by Earth s surface? 4) How would conditions on Earth be different without the greenhouse effect?
How Heat Is Transferred Heat is the transfer of thermal energy from a hotter object to a cooler one. Heat is transferred in three ways: radiation, conduction, and convection. Heat transfer by radiation Heat transfer by convection Heat transfer by radiation Heat transfer by conduction Heating the Troposphere Radiation, conduction, and convection work together to heat the troposphere. During the day, the sun s radiation heats Earth s surface. The land becomes warmer than the air. Air near Earth s surface is warmed by both radiation and conduction. However, heat is not easily transferred from one air particle to another by conduction. Only the first few meters of the troposphere are heated by conduction. Thus, the air close to the ground is usually warmer than the air a few meters up. Within the troposphere, heat is transferred mostly by convection. When the air near the ground is heated, its particles move more rapidly. As a result, they bump into each other and move farther apart. The air becomes less dense. Cooler, denser air sinks toward the surface, forcing the warmer air to rise. The upward movement of warm air and the downward movement of cool air form convection currents. Convection currents move heat throughout the troposphere. Checkpoint Questions: 1) How do the three types of heat transfers work together to heat the troposphere? 2) What is the major way that heat is transferred in the troposphere?
What Is Wind? Because air is a fluid, it can move easily from place to place. Differences in air pressure cause the air to move. A wind is the horizontal movement of air from an area of high pressure to an area of lower pressure. Winds are caused by differences in air pressure. Most differences in air pressure are caused by the unequal heating of the atmosphere. Convection currents form when an area of Earth s surface is heated by the sun s rays. Air over the heated surface expands and becomes less dense. As the air becomes less dense, its air pressure decreases. If a nearby area is not heated as much, the air above the less-heated area will be cooler and denser. The cool, dense air with a higher pressure flows underneath the warm, less dense air. This forces the warm air to rise. Measuring Wind Winds are described by their direction and speed. Wind direction is determined with a wind vane. The wind swings the wind vane so that one end points into the wind. The name of a wind tells you where the wind is coming from. For example, a south wind blows from the south toward the north. A north wind blows to the south. Wind speed can be measured with an anemometer (an uh MAHM uh tur). An anemometer has three or four cups mounted at the ends of spokes that spin on an axle. The force of the wind against the cups turns the axle. A meter on the axle shows the wind speed. Wind-Chill Factor On a warm day, a cool breeze can be refreshing. But during the winter, the same breeze can make you feel uncomfortably cold. The wind blowing over your skin removes body heat. The stronger the wind, the colder you feel. The increased cooling a wind can cause is called the wind-chill factor. Thus a weather report may say, The temperature outside is 20 degrees Fahrenheit. But with a wind speed of 30 miles per hour, the wind-chill factor makes it feel like 1 degree above zero. Wind Direction and Speed The wind vane on the left points in the direction the wind is blowing from. The anemometer on the right measures wind speed. The cups catch the wind, turning faster when the wind blows faster.
Local Winds Have you ever noticed a breeze at the beach on a hot summer day? Even if there is no wind inland, there may be a cool breeze blowing in from the water. This breeze is an example of a local wind. Local winds are winds that blow over short distances. Local winds are caused by the unequal heating of Earth s surface within a small area. Local winds form only when large-scale winds are weak. Sea Breeze Unequal heating often occurs along the shore of a large body of water. It takes more energy to warm up a body of water than it does to warm up an equal area of land. As the sun heats Earth s surface during the day, the land warms up faster than the water. As a result, the air over the land becomes warmer than the air over the water. The warm air expands and rises, creating a low-pressure area. Cool air blows inland from over the water and moves underneath the warm air, causing a sea breeze. A sea breeze or a lake breeze is a local wind that blows from an ocean or lake. Land Breeze At night, the process is reversed. Land cools more quickly than water, so the air over the land becomes cooler than the air over the water. As the warmer air over the water expands and rises, cooler air from the land moves beneath it. The flow of air from land to a body of water is called a land breeze. Sea Breeze Warm air rises Local Winds During the day, cool air moves from the sea to the land, creating a sea breeze. At night, cooler air moves from the land to the sea. Land Breeze Cooler air moves beneath warm air Warm air rises Cooler air moves beneath warm air
Global Winds Global winds are winds that blow steadily from specific directions over long distances. Like local winds, global winds are created by the unequal heating of Earth s surface. But unlike local winds, global winds occur over a large area. Recall how the sun s radiation strikes Earth. In the middle of the day near the equator, the sun is almost directly overhead. The direct rays from the sun heat Earth s surface intensely. Near the poles, the sun s rays strike Earth s surface at a lower angle. The sun s energy is spread out over a larger area, so it heats the surface less. As a result, temperatures near the poles are much lower than they are near the equator. Global Convection Currents How do global winds develop? Temperature differences between the equator and the poles produce giant convection currents in the atmosphere. Warm air rises at the equator, and cold air sinks at the poles. Therefore air pressure tends to be lower near the equator and greater near the poles. This difference in pressure causes winds at Earth s surface to blow from the poles toward the equator. Higher in the atmosphere, however, air flows away from the equator toward the poles. Those air movements produce global winds. Coriolis Effect As Earth rotates, the Coriolis effect turns winds in the Northern Hemisphere toward the right. Angle of Sun s Rays Near the equator, energy from the sun strikes Earth almost directly. Near the poles, the same amount of energy is spread out over a larger area. The Coriolis Effect If Earth did not rotate, global winds would blow in a straight line from the poles toward the equator. Because Earth is rotating, however, global winds do not follow a straight path. As the winds blow, Earth rotates from west to east underneath them, making it seem as if the winds have curved. The way Earth s rotation makes winds curve is called the Coriolis effect. Due to the Coriolis effect, global winds in the Northern Hemisphere gradually turn toward the right. As the picture towards the left shows, a wind blowing toward the south gradually turns toward the southwest. In the Southern Hemisphere, winds curve toward the left.
Global Wind Belts The Coriolis effect and other factors combine to produce a pattern of calm areas and wind belts around Earth. The calm areas include the doldrums and the horse latitudes. The major global wind belts are the trade winds, the polar easterlies, and the prevailing westerlies. Doldrums Near the equator, the sun heats the surface strongly. Warm air rises steadily, creating an area of low pressure. Cool air moves into the area, but is warmed rapidly and rises before it moves very far. There is very little horizontal motion, so the winds near the equator are very weak. Regions near the equator with little or no wind are called the doldrums. Horse Latitudes Warm air that rises at the equator divides and flows both north and south.latitude is distance from the equator, measured in degrees. At about 30 north and south latitudes, the air stops moving toward the poles and sinks. In each of these regions, another belt of calm air forms. Hundreds of years ago, sailors becalmed in these waters ran out of food and water for their horses and had to throw the horses overboard. Because of this, the latitudes 30 north and south of the equator came to be called the horse latitudes. Trade Winds When the cold air over the horse latitudes sinks, it produces a region of high pressure. This high pressure causes surface winds to blow both toward the equator and away from it. The winds that blow toward the equator are turned west by the Coriolis effect. As a result, winds in the Northern Hemisphere between 30 north latitude and the equator generally blow from the northeast. In the Southern Hemisphere between 30 south latitude and the equator, the winds blow from the southeast. For hundreds of years, sailors relied on these winds to move ships carrying valuable cargoes from Europe to the West Indies and South America. As a result, these steady easterly winds are called the trade winds. Prevailing Westerlies In the mid-latitudes, between 30 and 60 north and south, winds that blow toward the poles are turned toward the east by the Coriolis effect. Because they blow from the west to the east, they are called prevailing westerlies. The prevailing westerlies blow generally from the southwest in north latitudes and from the northwest in south latitudes. The prevailing westerlies play an important part in the weather of the United States. Polar Easterlies Cold air near the poles sinks and flows back toward lower latitudes. The Coriolis effect shifts these polar winds to the west, producing the polar easterlies. The polar easterlies meet the prevailing westerlies at about 60 north and 60 south latitudes, along a region called the polar front. The mixing of warm and cold air along the polar front has a major effect on weather in the United States.
90ºN Polar Easterlies Prevailing Westerlies blow away from the horse latitudes. 30ºN 60ºN Horse Latitudes Prevailing Westerlies Horse Latitudes are calm areas of falling air. Trade Winds Equator 0º Doldrums Doldrums are a calm area where warm air rises. Trade Winds Trade Winds blow from the horse latitudes toward the equator. Horse Latitudes Prevailing Westerlies 30ºS 60ºS W N E 90ºS Polar Easterlies blow cold air away from the poles. S A series of wind belts circles Earth. Between the wind belts are calm areas where air is rising or falling.
Polar Easterlies Cold air near the poles sinks and flows back toward lower latitudes. The Coriolis effect shifts these polar winds to the west, producing the polar easterlies. The polar easterlies meet the prevailing westerlies at about 60 north and 60 south latitudes, along a region called the polar front. The mixing of warm and cold air along the polar front has a major effect on weather in the United States. Jet Streams About 10 kilometers above Earth s surface are bands of high-speed winds called jet streams. These winds are hundreds of kilometers wide but only a few kilometers deep. Jet streams generally blow from west to east at speeds of 200 to 400 kilometers per hour, as shown below. As jet streams travel around Earth, they wander north and south along a wavy path. Jet Streams The jet streams are high-speed bands of winds occurring at the top of the troposphere. By traveling east in a jet stream, pilots can save time and fuel. Checkpoint Questions: 1) What is wind? 2) How is wind related to air temperature and air pressure? 3) What are local winds? 4) What causes local winds? 5) Compare the conditions that cause a sea breeze with those that cause a land breeze. 6) Briefly describe the three major global wind belts and where they are located.